1. Field of Invention
The present invention relates to a process for the automatic control of the thickness of extruded film.
2. Description of the Prior Art
Processes of such type are used both in flat sheet film extrusion as well as in blown film extrusion.
In modern extrusion plants, these processes usually comprise the following process steps:                Measurement of the thickness profile of the film just extruded with the help of a thickness-measuring probe that is moved along the surface of the film substantially perpendicular (x) to the conveying direction (z) of the extruded film. The thickness-measuring probe records for each measuring cycle (MZ) a thickness profile (P) of the film at least across parts of the expansion of the film perpendicular (x) to its conveying direction (z),        Transmitting the measured values to a control unit,        Storage of the measured values underlying the thickness profiles in a storage unit,        Provision of statistical values of the film thickness (5) using a computer (14), whereby the computer (14) takes into account measured values or information derived therefrom using a definite number (N) of measuring cycles (MZ) and, if necessary, provides measured values from recent and older measuring cycles with different weighting factors,        Determination of the deviations in the statistical values of the film thickness from a target value,        Generating control commands to a device for controlling the film thickness.        
The measuring devices outlined above are known from published prior art. Thus the patent specification DE 40 09 982 A1 proposes a capacitive sensor for measuring the thickness of the wall of a film tube. However, even other principles of measurement are used to measure the film thickness. For instance, even the measurements of the transmission behavior of beta radiation, gamma radiation, x-radiation and infrared radiation have proved to be useful. In blown film plants they are usually guided around the film tube that is just extruded. In flat film plants the sensor traverses across the width of the flat film extruded.
In this connection, statistical specifications about the development of the film thickness in terms of time generated using a computer serve to avoid an overmodulation or overshooting of the control process. For this purpose the computer takes into account measured values of a definite number of measuring cycles. The statistical values usually involve the computation of an average value or meridian. However, it is also possible to determine other statistical parameters.
Furthermore, it is possible to provide the computer with information derived from the measured values instead of the measured values themselves. This information derived from the measured values can be statistical values that result in the updated statistical values by taking into account the most recent measured values. Thus, for instance, an average value can be computed by feeding the average value of the last N measurements to the computer. The computer then only has to take into account the current measured value while computing the updated average value.
Information derived from measured values can also exist in the form of recorded “older” control commands that are adjusted on the basis of current measured values.
Measured values from a definite number of measuring cycles are used while computing the statistical values. Often measured values from a definite number of measuring cycles, for instance, the last N measuring cycles are used. Finally, in order to compute the average value the sum of the measured values at definite positions in x-direction is calculated and divided by N.
Furthermore, various methods can be used to determine the contribution of the measured values from different measuring cycles in the computation of the statistical values. Thus, for instance, greater importance can be accorded to the more recent, just recorded measured values than to the older values while computing the statistical values.
This variable weighting of the measured values, in short MWn, can be carried out with the help of weighting factors, here kn. In a statistical function that is similar to the average value computation, the sum would be computed as follows:Σ=k1MW1+k2MW2+ . . . +knMWn 
However, in this example, this sum would not be divided only by N, instead by the sumΣ=k1+k2+ . . . +kn 
in order to derive the statistical value similar to the average value. The information derived by the computer from the measured values and/or the statistical values is fed to a control unit that controls a device for controlling the film thickness. The thickness of the film can be controlled using different means. Thus, for instance, the width of the die gap or of the die ring can be varied in certain sections in order to be able to increase or decrease the throughflow of the melt at the desired places.
However, the control unit can also control the temperature of the melt by means of heating or cooling agents. Using the temperature it is possible to control in a targeted manner the viscosity of the melt. Should the viscosity of a melt be higher in one place than in other places, then the melted mass can “melt” more strongly in this place, thus causing the film to have a lesser thickness in this place.
Likewise the thickness of the film can be changed by stretching it in certain places. In this context, the property of the film that allows the already solidified and partially cooled film to stretch is exploited. The regions of the film that are stretched more strongly subsequently exhibit a lesser thickness than the regions of the film that are stretched using lesser strength. The force required for stretching the film is frequently made available by blow air. In this case the control unit controls the volume flow of the blow air in certain regions.
The measurement processes described above have proved to be useful in practice particularly in continuous operation. However, since in recent times a trend toward job orders of smaller sizes and thus toward a more frequent change-over of the film material can been noted, increasingly greater significance is attached to the automatic control of the film thickness at the start of the extrusion process.
However, film materials with unacceptable thickness tolerances and thus rejections have been produced using automatic control processes according to prior art during an important time span at the start of the extrusion process.
Therefore the objective of the present invention is to lower more quickly the deviations in the thickness of the film after the start of the extrusion process.